In a pinch, an apothecary might be expected to perform services that today would go to a surgeon, and sometimes they'd also serve as a midwife. Medical science was a little loosely defined in those days.

In an urban setting, the apothecary may well fulfill the same functions that a cunning man would in the countryside, and in a wainscot fantasy campaign, this is a good cover for a wizard.

Hope you've got clearly defined rules for poison in your game, at least if it's taking place in the standard fantasy setting. Buying a bottle of arsenic (or something far worse) is just "a quick pop down to the chemist", and pretty-much unregulated. Just claim it's to kill the rats in your walls, or the moles in your garden, and no one will give you any trouble.

Of course, if your rival is dead the next morning, the apothecary may have an inkling of who it did and how.

Here's a dirty little plotline for those who like to mess with their players a bit. Early in the campaign, you introduce an apothecary as a minor NPC, apparently just for color. Remind the PCs about Romeo & Juliet, etc. A few sessions later, you infest the PC's castle or HQ with some sort of vermin or vermin-like monster. Pretend you expect them to fight the monsters in a series of "bug hunts" and melee skirmishes. Chances are, one of the players will remember the "improvised" apothecary from three or four sessions ago, and go buy some rat poison. They score some XP and save themselves some trouble… or so they think. The next day, someone important in town is found dead of poisoning. The apothecary tells the sheriff that he just had an unusually large order for poison from the PCs. Enough to kill a man before he knew what hit him. The PCs will of course counter-accuse the apothecary of setting them up, but that might not matter if, say, the local apothecary did double-duty as midwife when the sheriff's children were born. It will take some clever thinking on the PCs part to resolve this - they won't want to skip town, especially not after having spent so much gold cleaning out the were-rat infestation at their headquarters. They can't just kill the apothecary, either, because that would "confirm" the sheriff's suspicions about them.

If potions are cheap and easy to make in your setting, then all the apothecaries will have them on the shelves. If they're a bit more trouble and effort, this is still probably the place to go (though a witch or alchemist might fill that role instead, then), it'll just cost more and they'll have to brew it up to order (so expect delays).

Again this will vary quite a bit depending on system and how magical the setting is. Some apothecaries will need to dabble in magic, others don't have any particular needs for special abilities beyond the the ability to brew herbal remedies.

Take a close look at the rules for poison in your game system. Is there a Resist Poison skill or special power? If so, you'll need it. Poisons tend to be pretty nasty, and you'll want any Feats or Edges that will improve your odds of surviving. Options that boost your saving throws are good ones. In some games poison does ability damage or something else other than eating away at your hit points, so make sure you've got the right stats to survive it.